1. Technical Field
The present invention relates to a method of exposing a resist of a photomask substrate to fabricate a photomask used for fabricating a semiconductor device, and more particularly, to an exposure method for correcting line width variation occurring during a development process in fabricating a photomask, and a recording medium in which the exposure method is recorded.
2. Description of the Related Art
Generally, in order to fabricate a photomask used for photolithography, a series of processes using electron beam lithography are performed as follows. First, a blank mask on which an opaque layer and an electron beam resist are sequentially deposited on a transparent photomask substrate made of quartz or glass is prepared. The electron beam resist is then exposed to an electron beam in a desirable pattern using an exposure apparatus. Subsequently, the exposed electron beam resist is developed using a development apparatus. The opaque layer is etched using the electron beam resist pattern as a mask, thereby forming an opaque layer pattern. Thereafter, the electron beam resist pattern is removed, thus completing a photomask.
In such fabrication of a photomask, an opaque layer pattern having a different line width from a desirable critical dimension (CD) is formed due to various factors in the fabrication, and the uniformity of a pattern line width decreases. If photolithography is performed using such photomask having a changing pattern line width or decreasing uniformity due to factors in fabrication, a pattern on a wafer also has changes in line width and decreases in uniformity. Accordingly, a photomask having a changing pattern line width or decreasing uniformity results in a semiconductor defect, thereby decreasing the fabrication yield. Consequently, fabrication cost increases. Therefore, it is necessary to analyze the causes of variations in pattern line width and to perform a corrective exposure.
Representative causes of variations in pattern line width occurring during fabrication are a fogging effect and a loading effect. In a fogging effect, an electron beam resist is diffusely exposed to electron beams reflected from the inside or the surface of an electron beam resist and the bottom of an objective lens of an electron beam emitter, which causes a line width to change. In a loading effect, a line width at a portion having a large loading density (i.e., an exposed area of an opaque layer underlying a removed electron beam resist) is greater than a line width at a portion having a small loading density when the opaque layer is etched.
Existing corrective exposure methods take into account fogging and loading effects. Fogging and loading effects are the causes of a variation in a line width occurring during an exposure process and an opaque layer etching process, respectively, among photomask fabrication processes. However, variation in line width occurring during a development process among the photomask fabrication processes has been neglected. Even if a photomask is fabricated by a corrective exposure method in consideration of fogging and loading effects, there is a limitation in increasing the uniformity of pattern line width. Accordingly, even if line width variation occurring during a development process is small, the line width variation exerts an influence which cannot be neglected in the fabrication of highly integrated circuits.
FIG. 1 is a schematic diagram of a development apparatus for explaining variation in line width during a development process.
Referring to FIG. 1, in a state in which a blank mask 20 is put in the development apparatus such that its exposed surface 25, that is, an electron beam resist exposed by an exposure process, faces upward, a developer 10 is ejected through a nozzle 15 downward. Here, the blank mask 20 is rotated, as denoted by the arrow 30, so that the developer 10 can be uniformly distributed. In other words, a spinning process is a general development process. However, when the developer 10 is ejected while the exposed blank mask 20 is rotated, line width uniformity of an opaque layer pattern changes due to variations in the flow velocity, relative flow rate, and heat of vaporization of the developer 10 on the exposed surface 25 of the blank mask 20. Since a spinning process is the development process, the pattern line width changes in a radial direction during the development process. A variation in the pattern line width during the development process decreases the processing margin of a wafer and weakens the cells at the edge of the wafer. Accordingly, the development of a corrective exposure method overcoming the above problems is desired.